Fallen Grass vs Cement Grey

How close are they?

ΔE 15.4, loose: different enough that you should pick on purpose. Cement Grey is noticeably lighter, more muted, warmer than Fallen Grass.

Can I use Cement Grey instead of Fallen Grass?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Cement Grey is noticeably lighter, more muted, warmer than Fallen Grass.

Can I use Fallen Grass instead of Cement Grey?

The same difference applies the other way round: Fallen Grass is noticeably darker, more saturated, cooler.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.